# Ironbug: Approximating a thermal labyrinth

We have a (1-storey) project where each zone has a thermal labyrinth underneath its floor. There is a natural ventilation mode where air enters the zone from the labyrinth via a floor grille and is exhausted through a solar fan in the ceiling; and, there is a mechanical ventilation mode with an HVAC system that has a separate outdoor air supply + VRF. The system can only be in one or the other state.

A wind engineer has done CFD studies on the thermal labyrinth to derive a model that predicts the air temperature at the labyrinth outlet (i.e. the inlet to the zone) according to the time of day and outdoor DBT.

The way I am trying to model the labyrinth is by making it part of the mechanical system in Ironbug. I am currently looking for a way to create a schedule for the outdoor air intake temperature of the air loop - does this option exist? My plan would be to use the EMS to create a control algorithm that defines the intake temperature as per the CFD-derived formula for natural ventilation mode, and to leave it at the outdoor DBT for the mechanical ventilation mode.

Does this make sense to you, and is this possible in Ironbug? Or, can you think of an easier way to model it? Any advice would be much appreciated.

Last year, I actually worked on Harvard’s new auditorium with a similar system (Mechanical Assisted Natural Ventilation).

Basically I created two hourly schedules: one for controlling the HVAC system, and one for natural ventilation via ZoneVentilation:WindandStackOpenArea ( https://bigladdersoftware.com/epx/docs/8-5/input-output-reference/group-airflow.html#zoneventilationwindandstackopenarea).

I don’t know what’s your purpose of this study, but for energy savings from NV, here are some items that you need to be careful of:

1. Make sure you have your envelope’s infiltration rate is what you expected.
2. You need to calibrate your energy model with NV only mode to compare the air flow rate with the results from CFD.
3. Create an hourly schedule for `Opening Area Fraction Schedule` for ZoneVentilation:WindandStackOpenArea.
4. Create another schedule for HVAC system.
5. In my case, I didn’t have a thermal labyrinth that creates a big delta T (outlet - inlet), I wasn’t worried about the air flow temperature that entering the occupied space. I am not sure what’s the best way to model this thermal labyrinth. Maybe you can consider create an artificial EPW file that with the air temperature replaced at hours when your building needs NV, and keep the rest hours original for HVAC. This hasn’t been validated, just one idea that came to my mind, similar to your plan, but might be easier to do with EPW file.

I don’t know if there is any HVAC component that allows you to control its temperature hourly basis.

Please let me know if this makes sense.

@MingboPeng,
Thanks for your suggestions. In our case, we expect there to be quite a large difference between inlet and outlet air temperature. Re customizing the EPW file, I would be a bit reluctant since I assume that this would also change the conductive heat flow through the constructions?

After much headache I have now used an EMS script that adjusts the cooling set point of each zone. The logic is:

``````if daytime:
if occupied:
HVACavailability = True
if Tin<23:
CLGsetpoint = 100
elif Tin<25:
CLGsetpoint = CFDregressionFunction(Tout)
else:
CLGsetpoint = 25
else:
HVACavailability = False
CLGsetpoint = 100
else:
if Tout[nextDayMaximum]>23:
HVACavailability = True
CLGsetpoint = CFDregressionFunction(Tout)
else:
HVACavailability = False
CLGsetpoint = 100
``````

That is, during the day, if the space is occupied and the indoor temperature is between 23 and 25C, use the regression function extracted from the CFD studies to estimate the outlet temperature and set this as the cooling set point (i.e. natural ventilation mode); above 25C, use this as the set point (i.e. HVAC mode). During the night, use night flushing if the next day will be warm.

I am still checking the results, but for now it seems to be working.